Conventionally, a spark plug is used for igniting an internal-combustion engine. An ordinary spark plug is comprised of: a metal shell radially surrounding and holding a circumference of an insulator in which a center electrode is accommodated in an axial bore; and a ground electrode in which one end thereof is joined to a front end of the metal shell and the other end thereof is bent towards a front end of the center electrode so as to face each other and form a spark discharge gap therebetween. Such a spark plug tends to be exposed at a high temperature because the ground electrode projects to a combustion chamber when the spark plug is attached to an engine head. Thus, since the heat load applied to the ground electrode becomes greater, an improvement in heat sinking ability (thermal conductivity) of the ground electrode has become highly demanded.
Japanese Patent Application Laid-Open (kokai) No. 2005-135783 discloses a ground electrode comprised of an electrode base material (e.g., nickel base alloy or the like) having corrosion resistance and oxidation resistance. A core material (e.g., Cu, Ag or the like) having an excellent thermal conductivity is embedded in the electrode base to promptly conduct heat generated during the engine drive to a metal shell. Generally, such a ground electrode is formed through an extrusion molding process to produce an integrated body where a cup-like electrode base material accommodates the core material therein. The thus-produced ground electrode is joined to the metal shell at a rear end portion thereof where a front end side in the extruding direction serves as a front end portion and a rear end side serves as the rear end portion. In the electrode base material, the core material is disposed so as to taper towards the front end side of the ground electrode.
However, since the power of an internal-combustion engine has been recently stronger, the heat load applied to a ground electrode has been greater in connection with a fuel combustion temperature in a combustion chamber. When a core material assumes a tapered shape towards a front end side of the ground electrode as mentioned above, the core material is disposed in a vicinity of an axis line and not near an outer circumference face in the front end portion of the ground electrode. Thus, heat that the front end portion of the ground electrode receives is unlikely to be promptly conducted to the metal shell, and the heat sinking ability of the ground electrode tends to be insufficient.
The present invention has been developed in view of the above problems, and provides a spark plug including a ground electrode which has an excellent heat sinking ability.